Trophic cascades

How predators structure ecosystems

Research at Oregon State University led by Bill Ripple and Robert Beschta has provided probably the most famous example of how terrestrial predators can change whole ecosystems. Ripple, Beschta, and colleagues studied what happened when wolves returned to Yellowstone National Park after an absence of 70 years. Wolves were reintroduced in the mid-1990s, and the population recovered to 14 packs – with almost 100 animals – by 2009. The recovering wolf population not only reduced numbers of their main winter prey, elk, but also changed their behavior.

Wolves can change ecosystems (Photo: Sandra Peterson)

The researchers found that in places where elk were at greatest risk from attack by wolves, such as in narrow stream valleys, aspens and willows – which previously had been browsed down by elk – were growing taller. Ripple and Beschta suggested that this was evidence for a “trophic cascade”: the presence of wolves was changing the behavior of the elk, and shifting their distribution within the landscape. In turn, the change in elk behavior was having an impact on the growth of trees, which themselves provided habitat for a wide range of other species.

The return of wolves may even change rivers. Ripple and Beschta found that after the extirpation of wolves in Yellowstone, riverbanks had eroded more rapidly, and their courses had become straighter and less meandering. With the return of wolves and the recovery of riverbank vegetation, the rivers in Yellowstone are returning to their meandering form, with benefits for a wide range of species, from willow flycatchers to beavers. Of course, the return of wolves is not the only factor: according to Ripple, both “top-down” (predator-driven) and “bottom-up” (plant-driven) processes shape Yellowstone’s complex and changing ecosystem. But the impact of wolves on willows is a timely reminder that everything in ecological systems is connected.

Since this early work on trophic cascades in Yellowstone, it has become increasingly apparent that predators – from sharks to tigers – have similar “keystone” influences in other ecosystems. Ripple and coauthors have recently called on world leaders to prioritize global conservation of predators.